[Magnifier - 18] Make #update() public

The CL adds the Magnifier#update() method in the public API. The method
is used to refresh the content of the magnifier, whenever this is
desired (usually when there is a chance that the magnifier content
became stale).

The initial plan was that this method would not be included in the
public API. This was relying on a feature request we made to the
graphics team, asking for support to have a callback called whenever the
surface the magnifier is attached to changes. This way, we could
refresh the magnifier content whenever the surface changes, without
requiring the user to call #update(). Once the feature request is
implemented (probably in Q according to the last discussion), we will be
able to deprecate #update().

Bug: 63531115
Test: atest CtsWidgetTestCases:android.widget.cts.MagnifierTest
Change-Id: I62c5794c3227e6a5d36d351c10d6bcf18e1d931a